The present invention relates to a device for promoting heat transfer between electrical components and a cooling plate wherein the electrical components and cooling plate are subjected to high levels of vibration or to multidirectional accelerations of a high magnitude, such as those vibrations or accelerations found in modern aircraft.
Heat transfer between two contacting surfaces occurs by conduction, convection and radiation. Of these three phenomena, the most effective is conduction. Conduction is the transmission of heat by the passage of energy from particle to particle in a material or between two mating materials. For maximum heat transfer by conduction between two surfaces it is therefore necessary to maximize the contact between the two surfaces. A variety of techniques have been used in the past for heat transfer by conduction across an interface between two surfaces. FIG. 1A shows a heat source 2 directly abutting a heat sink 4 with a dry interface between the heat source 2 and the heat sink 4. The direction of the flow of heat is indicated by arrows 6 and 8. Since conduction occurs only across those surfaces that are contacting, the heat transfer between two surfaces having no other interface material is entirely a function of the roughness and compatibility of the two dry surfaces. Air-filled interstitial voids exist where the mating surfaces of the heat source 2 and heat sink 4 do not directly contact and these voids do not effectively conduct heat. Typically, the thermal conductance across two dry surfaces is about 2.0 W/.degree. C.-in.sup.2 (watts per degree celsius per inch squared) when the pressure between engaging surfaces is 30 psi or greater.
When the configuration in FIG. 1A is subjected to severe vibration or high acceleration, there is essentially no change in the heat transfer rate between the heat source 2 and the heat sink 4 provided the pressure remains the same at 30 psi or more. However, totally independent of vibration or acceleration, the overall heat transfer of this configuration through conduction is poor.
FIG. 1B shows a heat source 2 next to a heat sink 4. However, in this figure, a layer 10 of aluminum silicate paste or alternatively of grease having a thickness of about 0.003 inches is situated between the heat source 2 and the heat sink 4 such that the interstitial spaces found in FIG. 1A are filled with grease and therefore heat transfer through conduction may occur across the entire surfaces of the heat source 2 and heat sink 4. In this case, the thermal conductance is approximately 5.6 W/.degree. C.-in.sup.2 and is independent of mating surface pressure.
When the configuration in FIG. 1B is subjected to high levels of vibration or acceleration, while the heat source 2 and the heat sink 4 may be held securely, the layer 10 of paste or grease eventually works its way from between the heat source 2 and heat sink 4 until the heat transfer of the configuration is reduced to a level comparable to that of the configuration shown in FIG. 1A.
FIG. 1C shows a heat source 2 and a heat sink 4 with a solid layer 12 of an elastomer material having a thickness of about 0.01 inches and contacting the surfaces of both of the heat source 2 and the heat sink 4. While this configuration is very resistant to high levels of vibration and acceleration, the thermal conductance is only about 4.4 W/.degree. C.-in.sup.2 at moderate pressures of 10 to 30 psi.
FIG. 1D shows an arrangement by which the heat source 2 and the heat sink 4 are both contacted by a common layer 14 of liquid metal having a thickness of about 0.01 inches. The thermal conductance through the layer of liquid between these two surfaces is about 52.6 W/.degree. C.-in.sup.2. Clearly this arrangement provides for a highly desirable heat transfer rate that is approximately ten times greater than that of any of the previous methods discussed. However, while such a heat transfer rate is highly desirable when compared with all of the techniques discussed, the configuration shown in FIG. 1D is also the most susceptible to high levels of acceleration or vibration since the layer 14 of liquid metal may freely escape from between the heat source 2 and the heat sink 4.
One object of this invention is to provide an improved heat transfer configuration between two mating surfaces having a heat transfer comparable to that of liquid between the surfaces.
Another object of this invention is to provide a thermal coupling to promote heat transfer that is capable of withstanding very high accelerations or severe vibration.
Another object of this invention is to provide a thermal coupling to promote heat transfer wherein the coupling is relatively simple to install and may be used in a mass production system.
U.S. Pat. No. 4,384,610 by Cook et al., entitled "Simple Thermal Joint", discloses a mechanically breakable thermal joint having a low melting point metallic material formed around a screen-like cellular structure. Heat transfer occurs between a heat source and a heat sink via the metallic material which melts and provides an intimate contact by filling any interstitial voids between the heat source and the heat sink surfaces. The screen-like cellular structure functions to promote retention of the molten metal between the two surfaces. However, this screen-like structure does not continue past the perimeter of the heat source and heat sink surfaces. When the configuration is subjected to high acceleration or vibration levels, molten metal escapes through the exposed edges of this configuration because the molten metal is not completely contained between the two surfaces.